In general, the term “organic light emitting phenomenon” refers to a phenomenon in which electric energy is converted to light energy by means of an organic material. Particularly, when an organic layer is disposed between an anode and a cathode and then a voltage is applied between both electrodes, holes from the anode and electrons from a cathode are injected into the organic layer, the holes and the electrons injected are combined together to form excitons. Further, when the excitons drop to a ground state, lights are emitted.
In addition to the above-described organic light emitting phenomenon in which light emission is made by recombining of charges injected from both electrodes, there is another mechanism in which holes and electrons are not injected from external electrodes but are generated by an amphoteric charge-generating layer under the application of alternating current voltage, as in the case of an inorganic thin film light emitting device, and the holes and electrons move to an organic thin film layer, resulting in light emission (Appl. Phys. Lett., 85(12), 2382-2384).
Since Pope, Kallman and Magnate have found electro-luminescence in anthracene single crystal in 1963, active research and development into OLEDs (Organic Light Emitting Devices) have been made up to now. Recently, organic light emitting devices have been used in various applied products such as flat panel display devices and lighting devices.
In order to manufacture more efficient organic light emitting devices, an attempt has been made to manufacture an organic layer in the device in the form of a multilayer structure instead of a monolayer structure. Most of currently used organic light emitting devices have a structure in which an organic layer and electrodes are deposited. The organic layer generally has a multilayer structure including a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer.
It is known that organic light emitting devices are characterized by high brightness, high efficiency, low drive voltage, color changeability, low cost, etc. However, in order to meet such characteristics, each layer forming an organic layer in a device (for example, a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer) must be formed of more stable and efficient materials.